Questions answered

QUESTION: An article on the Web, “Global Warming: A Closer Look at the Numbers,” states that the amount of manmade CO2 in the atmosphere is 0.28 percent. Can this be true?

– Jerry Allen, San Diego

ANSWER: No. The global atmospheric concentration of carbon dioxide increased from 280 parts per million before industrialization to 380 ppm today. Some have claimed the increase is due to release of carbon dioxide by the oceans, but three lines of evidence prove this claim is nonsense.

First, the increase in atmospheric carbon dioxide is considerably less than what humans have produced. The oceans have absorbed much of the “missing” carbon dioxide, and have acidified as a consequence.

Second, isotopes of carbon and oxygen in atmospheric carbon dioxide are consistent with the increase being from manmade sources.

Third, the atmospheric oxygen concentration has declined since preindustrial times, which is consistent with the burning of fossil fuels, but the opposite of what would happen if the oceans were releasing gasses.

The article you mention actually claims that 0.28 percent of the greenhouse effect is caused by humans. To arrive at this number, the authors overestimate the influence of water vapor, which is mostly natural in origin.

The Earth's total greenhouse effect, including clouds, is 155 watts per meter squared. This makes Earth 33 degrees Celsius warmer than it would be without an atmosphere. According to the Intergovernmental Panel on Climate Change, the current net warming impact of human activities is 1.6 watts per meter squared, about 1 percent of the total greenhouse effect.

Sherry Seethaler is a UCSD science writer and educator. Send scientific questions to her at Quest, The San Diego Union-Tribune, P.O. Box 120191, San Diego, CA 92112-0191. Or e-mail
sseethaler@ucsd.edu
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